Development of an advanced generation breeding strategy for Eucalyptus Nitens (Deanne and Maiden) Maiden.

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Abstract

The objective of this study was to develop and implement an advanced generation breeding
programme at the Institute for Commercial Forestry Research (ICFR) to manage and
integrate the many and disjunct breeding and production populations of Eucalyptus nitens
established by various entities over the past 30 years at multiple sites in South Africa. To
develop such a breeding strategy, a good understanding of the population genetics, and the
underlying assumptions made by tree breeders about the species, was needed.
Eucalyptus nitens is an important forestry species grown for pulp and paper production in the
temperate, summer rainfall regions of South Africa. A tree improvement programme has
been ongoing at the ICFR for three decades. The measurement and statistical analysis of
data from eight F1 trials established during the 1980s and 1990s have enabled
characterisation of the ICFR’s breeding population. Provenance testing showed that the
more northerly New South Wales (Australia) Eucalyptus nitens provenances of Barren
Mountain and Barrington Tops are distinctly better suited to growth in South Africa than the
southern New South Wales provenances and the Victorian provenances, Penny Saddle and
Bendoc. Generally, the species was not badly affected by Coniothyrium canker. High Type B
genetic correlations for all sites pairs, except one comparison, ranged from 0.75 to 0.99 for
diameter at breast height at 76 to 113 months, indicating very little, or no, genotype by
environment interaction for diameter at breast height for the genotypes tested in the F1
generation. Narrow sense heritability estimates ranged from 0.01 to 0.34, indicating that the
species provides a breeding opportunity for improvement of diameter growth. High genetic
correlations of greater than 0.90 between diameter measurements at 52 to 62 months after
establishment and diameter measurements at 94 or 113 months were found, indicating that
selections can be made reliably at five or six years. Diameter measurements at both 60
months and full rotation (94 to 113 months) were highly correlated with the final height
measurements in these trial series (rg > 0.71 and > 0.83, respectively). Predicted genetic
gains for the F2 over the F1 generation were highest in the trials at Goedehoop and Arthur’s
Seat, with predicted increases in diameter at breast height of 3.07 cm (17.1%) and 3.17 cm
(20.7%), respectively, at full rotation.
Genetic improvement in the species has been slower than anticipated due to delayed and
infrequent flowering and seed production. Three genetic gain trials were established, firstly,
to quantify the gains that have been made in the first generation of improvement in the
breeding programme; and secondly, to establish whether a number of seed source and
orchard variables influence the performance of the progeny. These variables were: the
number of flowering trees in the seed orchard, year of seed collection, seed orchard origin
and composition of seed orchard seed bulks. Diameter at breast height and tree height were
measured in the trials at between 87 and 97 months after establishment, and timber volumes
and survival were calculated. Improved seed orchard bulks performed significantly better
(p < 0.01) than unimproved controls in the field trials, and genetic gains ranging from 23.2 to
164.8 m3ha-1 were observed over the unimproved commercial seed. There were significant
differences (p < 0.01) in progeny growth between the levels of flowering, with higher levels of
flowering (R 40 %) producing substantially greater progeny growth than lower flowering levels
(S 20 %). The seed orchard origin had no effect on progeny growth in this trial series. This
suggests that seed collected from any of the four seed orchards tested will produce trees
with significant improvement in growth.
Various scenarios investigating a range of assumptions were developed and used to predict
genetic gain in the F2 populations. These were compared with realised gains achieved in the
genetic gain trials. The family nested within provenance scenarios proved to be closer to
realised gain than the family across provenance predictions. Two scenarios were used for
family nested within provenance: Firstly, actual flowering for family nested within provenance;
and secondly, estimated flowering after a 30% roguing of poor families. For both scenarios, a
coefficient of relationship of 0.33 predicted gains closest to the realised gains. Indications
were that the effects were additive, and that very little or no heterosis had occurred. The
statistical information suggested that outcrossing in the seed orchards was > 80%. This study
provides an objective and quantitative assessment of the underlying assumptions used for
estimating genetic parameters, and predicting gain in this population of Eucalyptus nitens.
At the same time that genetic gain trials were established, F2 trials were planted, using
seedlots collected from F1 seed orchards. Analysis of the two F2 trials showed that realised
gains for diameter at breast height at 87 months were close to the predicted values and
ranged from 1.02 cm to 1.90 cm. Two exceptions were the sites at Helvetia and Babanango,
where gains were under- and over-predicted, respectively. Realised heritability estimates, which are related directly to the realised gain and the actual selection intensities used in the
seed orchards, reflected this trend. Estimation of breeding values allowed for selection of
elite individuals in top families. Both grand-maternal provenance origin and F1 maternal
effects were significant in the F2 trials. A Type B genetic correlation of 0.61 for diameter at
87 months indicated the possible presence of genotype by environment interactions for the
two F2 sites. A low narrow sense heritability estimate of 0.06 for diameter at breast height at
87 months at one F2 site indicated that more emphasis should be placed on family
information rather than individual information at this site. A heritability estimate of 0.17 for
diameter at breast height at 87 months at the second site, however, indicated that further
improvement is possible in this population of Eucalyptus nitens.
Modelling of predicted genetic gain using various breeding strategy scenarios can be a
useful tool in assisting with the decision on which strategy or management plan will deliver
the most genetic gains per unit time. Such modelling, using the parameters established in the
first part of the study, played an important role in developing the advanced generation
breeding strategy for Eucalyptus nitens. In addition, the modelling exercise highlighted
various management options which could be used to increase gains in the existing
production populations or orchards. Indications are that additional roguing of 1) existing
Clonal Seed Orchards based on results of F2 trials (i.e., backward selection); and 2) F1
Breeding Seed Orchards based on stricter provenance selection, will markedly increase the
quality of the seed produced from these orchards within one season. This study also
highlighted the importance of shortening the breeding cycle in Eucalyptus nitens, particularly
in view of the delays caused by reticent flowering and seed production in the species.
The information and understanding gathered from this study led to the development of a
proposal for an advanced generation breeding strategy in Eucalyptus nitens. This proposal
uses parental reconstruction of open-pollinated progeny to secure pedigree information of
forward selections, thus combining the benefits of increased genetic gain with a shortened
breeding cycle. Recommendations on the management and adaption of current production
populations to increase gains have been made, because establishment and management of
improved material in seed orchards to ensure a sustainable supply of improved seed to the
South African forestry industry, is a key objective of the ICFR Eucalyptus nitens breeding
programme.

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